This invention pertains to a catalytic isomerization process for light hydrocarbons such as pentane, hexane and heptane, wherein a hydrocarbon stream contacts a catalyst composition containing an ultrastable, large pore, crystalline zeolitic aluminosilicate material and a Group VIII metal, metal oxide or sulfide under suitable isomerization conditions.
Natural straight run gasoline, i.e., naphthas, contain chiefly normal paraffins such as normal pentane and normal hexane, which have relatively low octane numbers. Representative octane numbers of pentanes and hexanes are set forth below in Table I.
TABLE I ______________________________________ Octane Numbers of Pentanes and Hexanes Octane Number Research Motor Paraffin Clear Clear ______________________________________ n-Pentane 61.7 61.9 2-Methylbutane 92.3 90.3 2,2-Dimethylbutane 91.8 93.4 2,3-Dimethylbutane 103.5 94.3 2-Methylpentane 73.4 73.5 3-Methylpentane 74.5 74.3 n-Hexane 24.8 26.0 ______________________________________
It is therefore of great importance to convert these low octane components to their higher octane counterparts in order to supply the present and future requirements for production of gasoline with reduced lead content or no lead content at all. Accordingly, a considerable number of materials have been proposed as catalysts for isomerization of hydrocarbons in the petroleum industry and there is need for more effective catalysts.
It is well recognized in the prior art that the isomerization reaction is unusual in that the thermodynamic equilibrium between the various isomers is the most important feature in determining the catalyst which can be applied most successfully to the process. This is because the equilibrium concentration is very dependent upon the reaction temperature. The effect of temperature is particularly apparent in the case of light hydrocarbons such as the hexanes.
It is well-known that if the isomerization reaction is carried out at high temperatures, the doubly branched isomers are much less favored than singly branched isomers or n-hexane whereas at temperatures below 300.degree. F. there is a rapid increase in the equilibrium concentration of the high octane isomer 2,2-dimethylbutane. (J. A. Ridgway, Jr. and W. Schoen, ACS Symposium, Div. of Petroleum Chemistry, Boston, April 5-10, 1959, A-5-A-11) Thermodynamic equilibrium curves therefore provide criteria as to catalysts which provide near equilibrium conversions at the lowest possible reaction temperature. FIG. 1 shows the composition-temperature equilibrium curves of the vapor phase hexane isomers as determined by Ridgway and Schoen. The relative mole percent equilibrium of each component of the stream indicates the relative catalyst activity. The curves indicate the low intermediate temperature range of from about 350.degree. to about 650.degree. F., with a mid-point temperature of about 550.degree.-600.degree. F., combines the advantages of high isomer yield with lower investment in equipment.